1. Field of the Invention
This invention relates to improved composite materials containing cellulosic pulp fibers dispersed in a polymeric matrix material. The invention also relates to melt-blending and extrusion methods of making these composites and methods of using the same in injection molding applications.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
In the plastics industry, fillers and reinforcement materials are typically used to improve the properties of plastics. The addition of such materials can improve properties such as conductivity, strength, modulus values, notched impact resistance, etc.
Glass fibers are the most used reinforcement material for thermosets and thermoplastics. Glass fibers impart high strength, dimensional stability, and heat resistance to a plastic composite. Although glass fibers achieve desirable reinforcing properties, glass fibers are costly, abrade processing equipment and increase the density of the plastic systems. In certain applications, these disadvantages outweigh the advantages of using glass fibers as a reinforcement additive.
Cellulosic pulp materials have been evaluated as fillers for plastics in the past. Klason, et al., "Cellulosic Fillers for Thermoplastics", Polymer Composites, (1986); Klason, et al., "The Efficiency of Cellulosic Fillers in Common Thermoplastics. Part 1. Filling without processing aids or coupling agents", Intern. J. Polymeric Mater., Volume 10, pgs. 159-187 (1984); Snijder, et al., "Polyolefins and Engineering Plastics Reinforced with Annual Plant Fibers", The Fourth International Conference on Wood Fiber-Plastic Composites, pg. 181-191.
Cellulosic pulp materials have relatively low densities (approximately 1500 kg/m.sup.3) and result in reduced wear on the processing equipment compared to glass and mineral materials [e.g., the density of wollastonite, a mineral fiber, is 2900 kg/m.sup.3 ; the density of E(electrical) glass fiber is 2500 kg/m.sup.3 ]. However, prior investigations of the use of wood cellulosic pulps or raw lignocellulosic resources (e.g., wood flour, bagasse) in polymeric materials such as thermoplastics found that a pronounced discoloration of the composite material occurred with the use of these materials at temperatures above 200.degree. C. Furthermore, the use of such pulps were found to cause significant off-gasing and disadvantageous odors, principally due to impurities such as lignin. Moreover, previous studies have also found that at temperatures above 200.degree. C. the cellulosic fibers themselves had poor reinforcing properties compared even to ground wood and cellulose flours [Klason, et al., Intern. J. Polymeric Mater., Volume 10, p. 175 (1984)]. These disadvantageous results directed previous research efforts to the use of cellulosic materials in polymers having melting temperatures below 200.degree. C.such as polypropylene and polyethylene (melting temperatures below 180.degree. C.), and away from higher melting temperature materials.
It would be desirable to provide an improved reinforcement filler for use in polymeric materials such as thermoplastics where the filler has a lower cost, lower density, increased reinforcing characteristics, reduced abrasiveness, and the ability to be processed at high temperatures (e.g., above 200.degree. C.).